1. Field of the Invention
The present invention relates to an image forming apparatus that includes a fixing unit, and a control method therefor.
2. Description of the Related Art
The image forming apparatus using an electro photography method develops an electrostatic latent image, which is formed on a photo conductor by laser radiation, by development agent supplied from a development unit to visualize as an image of the development agent, transfers the image of the development agent to a recording paper, and fixes it by a fixing unit. In order to fuse the development agent on the record paper and to fix it to the recording paper, the fixing unit is heated to high temperature. As a heat source for that, an induction coil, a halogen heater, etc. are used. The electric energy applied to the fixing unit is usually controlled based on a temperature value detected by a detector such as a thermistor so that the fixing unit keeps a predetermined fixing temperature at about 200 degrees centigrade.
Moreover, in order to secure a fixing ability during a print operation, it is necessary to apply an electric power to compensate a heating value lost by passing the recording paper. Therefore, the electric power required by the fixing unit to secure the fixing ability increases as the number of the recording papers passing through the fixing unit per a unit time increases with improvement in the speed of the image forming apparatus. In a color image forming apparatus, since the total amount of the development agent applied onto the recording paper in an overlapped fashion increases, the more electric energy is required to fuse and fix. The image forming apparatus is provided with various parts as loads that consume the electric power such as a motor for conveying a paper and a semiconductor laser for exposing the photo conductor, besides the fixing unit.
However, a usable electric power is restricted by an environment of an electric power source to which the image forming apparatus is connected. For example, the maximum usable electric power of a general plug socket in Japan is 100V/15 A, i.e., 1500 W.
Therefore, conventionally, the maximum electric power consumptions of the respective loads including the fixing unit have been estimated, and the apparatus is designed so that the sum total of the maximum electric power consumptions does not exceed a power supply capacity (for example, 1500 W). Although this design method is based on the sum total of the maximum electric power consumptions of the respective loads, the electric power consumption of the image forming apparatus becomes lower than the power supply capacity during an actual operation. Therefore, the usable electric power is not used efficiently.
In contrast to such a method, Japanese laid open patent publication (Kokai) No. S58-105180 (JP S58-105180A) discloses a technique to control a possible electric power supplied to the fixing unit based on a permissible maximum electric power that is determined in consideration of the power supply capacity. With this technique, an electric current sensor that detects electric current amount flowing into the image forming apparatus is provided, and a total electric current consumption flowing into the image forming apparatus is detected. A temperature of the fixing unit is detected and is compared with a predetermined value, and the electric current supplied to a heat source heater is controlled based on the compared output. And then, the electric power supplied to the fixing unit is controlled so that the total electric power consumption of the image forming apparatus is not larger than the power supply capacity.
However, the technique disclosed in Japanese laid open patent publication (Kokai) No. S58-105180 (JP S58-105180A) has the following disadvantages. That is, when the electric current sensor breaks down and an improper detection value thereof is used to control, a fixing temperature may excessively rise due to an oversupplying of an electric power to the fixing unit, or a fixing temperature may fall due to an insufficient electric power. This causes problems such as an output of an abnormal image due to poor fixing and an increase in a down time of the apparatus, as a result.